Title text: Later: 'Why is there a book hovering over the trash can?'
The joke here is that the "How to Juggle" book assumes that there is gravity and momentum in the place where you are juggling. It seems like there is no gravity or momentum because when Cueball throws the juggling balls up in the air, they don't come down, and also don't continue the trajectory that they had when they left his hands.
The title text tells us that someone else comes along and sees that the juggling book is still hovering over the trash can and never went into it.
While it is possible to reach zero gravity (or at least microgravity), there is no place in our universe where objects with mass have no momentum. Some possible explanations might be that Cueball is outside of our universe, he has just discovered something that's theoretically impossible, or he is just dreaming, or Randall has taken comedic license on the "momentum" part for the sake of the joke.
Also, because Cueball jumps and returns to the ground (panels 6 & 7), it would appear that the effect is limited to the balls and the book, and does not extend to Cueball himself.
- [The panel shows a close up of Cueball reading a book. The book is called "How To Juggle" and has a picture of a person juggling on the cover.]
- [The view now shows the entirety of Cueball. A book is splayed on the floor behind them, and he is holding some juggling balls.]
- [Cueball throws the juggling balls in the air.]
- [He lowers his arms to prepare to catch the balls. The balls are still hovering in mid-air.]
- [Cueball now stands with his arms by his sides. The balls have not moved and are still suspended in mid-air.]
- [Cueball jumps, trying to grab the lowest ball. He can't reach.]
- [Cueball scratches his head and stares at the still floating juggling balls.]
- [Cueball throws the book into a trash can.]